The present invention relates to a method and circuit for evaluating output signals from one or more sound pickups of a monitoring system, particularly an open-air monitoring system, and for generating an alarm signal.
It is generally known to protect objects by means of alarm systems including sensors to detect signals which actuate an alarm and which are generated, for example, by a person approaching the object to be protected. Such sensors may be, for example, sound pickups, e.g. body vibration pickups, water vibration pickups, etc., photoelectric sensors, or the like. The signals detected by these sensors are processed by means of appropriate circuitry and are used to actuate an alarm, usually when a given threshold is exceeded in one direction or another.
All known alarm systems also have in common that they can be made to emit false alarms as a result of interference signals. This is particularly likely in the case of open-air monitoring systems in which various environmental influences produce a number of interference signals. It is known to attempt to eliminate such interference signals by specific circuitry measures and in particular to prevent such interference signals from exceeding in amplitude the signals to be detected. There thus occurs, in the prior art alarm systems, an amplitude evaluation which, to attain a relatively high detection probability, brings about a disadvantageously high number of false alarms.